Electric hand-held power tools, for example, impact screw drivers, drills, angle grinders, jigsaws, circular saws, or planers for the needs of craftsmen or handymen usually have either an AC motor or a DC motor as the drive motor. While the former is generally supplied with alternating current from the grid via a power cable, the electrical power for supplying the DC motor generally comes from a so-called battery back, a rechargeable battery in a housing which may be coupled to the housing of the hand-held power tool, which is electrically connected to the current supply lines of the DC motor during the coupling of the two housings.
Battery packs of this type are basically conventional and have rechargeable batteries, generally a plurality of battery cells connected in parallel and/or in series. In relation to this application, a battery pack is thus understood as a battery packet, which is made up of multiple electrically interconnected battery cells and may store electrical energy, deliver the energy necessary for operating the hand-held power tool, and is accommodated interchangeably in a chamber, an interface, or the like of a hand-held power tool. The coupling of the battery pack to the hand-held power tool is carried out by plugging or inserting the battery pack into a complementary plug-in socket of the device housing, the battery pack interacting with the device housing of the hand-held power tool in such a way that upon coupling the battery pack to the device housing, the power tool is electrically connected and mechanically locked to the battery pack. The electrical contacting is carried out mostly in the area of the locking device.
It is generally provided to connect the battery cells or battery cell blocks to one another in series. In part, so-called conductive plates are used here as cell connectors for the series connection, and connect the respective complementary poles of the battery cells to one another. The cell connectors are usually fixed on the poles of the battery cells by a welding or soldering process, resistance welding, resistance spot welding in particular, being preferably used. During the manufacture of the cell connectors, materials like nickel or copper alloys and steel are preferably used.
In resistance spot welding, the join partners are situated in a lap joint and a welding current is applied in a locally limited area, the Joule heating which accompanies this causing a melting of the join partners in the area of a so-called weld nugget. Upon solidification of the melt, a punctiform, integrally bonded connection remains between the join partners.
The welding current flows between two generally rod-shaped electrodes, which are situated on the join partners in such a way that the welding current is transmitted in the joint between the join partners. The process is designed here in such a way that the largest electrical resistance, and thus the greatest heating, occurs in the area of the joint. In this way, it is achieved that the join partners melt first in the area of the joint or the weld nugget.
The process may be better controlled, if the largest resistance along the path, which the current travels through the join partners, is more distinct in the area of the weld nugget, and the current flows with greater concentration through this point. Undesirable effects which oppose an efficient process control result, for example, from the contact resistance prevailing between the electrodes and the join partners, and from the so-called parasitic shunt which flows through the join partners between the electrodes, but not through the weld nugget. This portion of the current does not contribute to the Joule heating of the weld nugget, and in this sense is lost to the welding process, and may, in the worst case, lead to damage at unforeseen points.
In general, the fixing of a cell connector on a pole of a battery cell is carried out in such a way that both electrodes are applied to the cell connector which in turn contacts the respective pole of the battery cell. In order that a sufficiently high proportion of the current flows through the joint between the cell connector and the battery pole and that the entire welding current is not lost in the shunt through the cell connector, the conductivity of the cell connector, on the one hand, and the contact resistance between the cell connector and the battery cell pole, on the other hand, must be adjusted to one another. It has thereby proven to be problematic that the cell connector is to have a preferably good conductivity between the battery cells in the installed state, as this ensures that the parasitic shunt becomes large relative to the usable welding current. A cell connector, intended for use in the battery pack, having a preferably low electrical resistance thus generally opposes an efficient and stable resistance spot welding process.